1. Field of the Invention
The present invention relates generally to a presence detector having a passive infrared sensor for detecting the presence of stationary persons in a room.
2. Description of the Related Art
In modem presence detectors, the passive infrared sensor is equipped with a pyro-sensor for detecting thermal radiation and a structure for focusing the thermal radiation from the room being monitored, which impinges upon the pyro-sensor. The signal of the pyro-sensor detects movements of heat sources which differ from the ambient temperature in the room being monitored (see EP-A-0 303 913, for example). Today, such passive infrared sensors are obtainable in many versions and at favorable prices. However, conventional infrared sensors are either unable, or poorly able, to detect stationary persons working, for example, at a PC. Therefore, passive infrared sensors have only limited use in presence detectors in an office environment. Furthermore, it is not possible to determine the level of occupancy of a room with the passive infrared sensors currently available on the market.
If, instead of a conventional passive infrared sensor, a passive infrared sensor array using so-called thermopile technology is employed (see European patent application 98 115 476.8), then the presence detector can indeed detect stationary objects which exhibit a temperature difference vis-a-vis the environment, and also respond to warm objects such as heaters, computers or locations exposed to sunlight. However, for sufficiently high resolution, these sensor arrays are currently still very expensive. Accordingly, an improved presence detector which can detect stationary persons that is manufactured at competitive price is required.
An object of the present presence detector is to reliably detect stationary persons and to distinguish them from warm objects in the room.
Another object is to provide a cost effective presence detector that can determine the level of occupancy of a room.
These and other objects are achieved with a presence detector of the type referred to at the outset, in that an image sensor operating in the visible spectral range, and an electronic evaluator for the evaluation of the image information, are provided in addition to the passive infrared sensor. The signals of both sensors are then evaluated and the passive infrared sensor signal is used to activate the image sensor. If necessary, it is also used to switch on the room lighting.
The monitoring of the room for the presence of persons is primarily carried out by the image sensor. The passive infrared sensor is mainly used to activate the image sensor and to switch on the lighting of the relevant room if this is necessary. This arrangement has an added advantage that the image sensor can always operate under adequate light conditions. Since the image sensor operates in the visible spectral range, it cannot xe2x80x9cseexe2x80x9d in the dark, and with insufficient brightness it has to rely on suitable lighting.
In one embodiment, the presence detector includes a rapid reaction passive infrared sensor to switch on the lighting of the relevant room when entered by a person. Due to the use of the rapid reaction passive infrared sensor, which switches on the room lighting as soon as a person enters a room in which there are inadequate lighting conditions, the image sensor is always present in an adequately illuminated room and no additional lighting adjustments are needed for the proper functioning of the image sensor.
In another embodiment, the image sensor is provided to detect the presence of persons in the relevant room.
The image sensor can take the form of a complimentary metal-oxide semiconductor (CMOS) image sensor or is an active pixel sensor.
The image sensor scans the room to be monitored by imaging, digitizing the image and storing it as a reference image in a memory. The use of an active pixel sensor, which is characterized by a very low power consumption, makes it possible to access individual pixels. If the active pixel sensor consists of a sufficiently large number of pixels, raster scanning is obtained in which even small movements, such as hand movements, for example, can be detected. In the active state of the presence detector, the image sensor generates an image of the monitored room at intervals of fractions of a second, stores these images for a specific time and compares them with the reference image and/or with each other.
In a further embodiment of the presence detector, the image sensor is designed to measure the ambient brightness. In this regard, the image sensor can have an arrangement for measuring the ambient brightness, such as a photo-diode operationally coupled with the image sensor.
In other embodiments, the passive infrared sensor switches on the lighting when this is actually required, and the lighting can be switched off by the image sensor when, because of adequate brightness, it is no longer required.
In a further preferred embodiment of the presence detector, the image sensor""s visual range is subdivided into several partial areas, and a separate evaluation of the sensor signal for each partial area takes place during the evaluation of the image sensor signal. This embodiment has the advantage that the occupancy of the room, that is to say, the number of persons in it, can be at least estimated and used, for example, for the control of heating/ventilation/air-conditioning as required.
In a yet another embodiment of the presence detector, the image sensor has an optical system for displaying several partial areas on the same display area. This results in multiple use of the image sensor and an increase in resolution, allowing an image sensor of a lower resolution to be used, which leads to a corresponding cost reduction for the image sensor.
The invention further concerns a use of the presence detector for the xe2x80x9con-demandxe2x80x9d activation and/or control of conditioning facilities of a room. Conditioning facilities are understood to be facilities for influencing the ambient conditions prevailing in the respective room, such as room brightness or climate. At least for reasons of energy savings, there is a requirement to regulate ambient conditions, in particular to switch off or reduce the lighting, heating, ventilation, and air-conditioning in empty rooms and to switch them on or to adjust them to normal operation as soon as somebody enters the room. Moreover, xe2x80x9con-demandxe2x80x9d control means the control of heating/ventilation/air-conditioning according to the number of persons located in a room.
Accordingly, the signal of the passive infrared sensor can be used to activate the image sensor and, if necessary, to switch on the room lighting. Additionally, the ambient brightness may be measured and used to determine whether to switch the room lights off. The signals of both sensors are used to control the heating, ventilation, and/or air-conditioning of the room. The image sensor signal is additionally used to switch-off the lighting.